William Burnside and the Mystery Letter

Relatively little is known about the correspondence of William Burnside, a pioneer of group theory in the UK. There are only a few dozen extant letters from or to him, though they are not without interest. However, one of the most noteworthy letters to or at least about him, in that it had a special mention in his obituary in the Proceedings of the Royal Society, has not been positively identified. It's not clear who it was from or when it was sent. We'll look at some possibilities.